Loudspeaker diaphragm support



June'24, 1958 H. scHoENGoLD 2,840,177

LOUDSPEKER DIAPHRAGM SUPPORT Filed July 28, 1955 2 Sheets-Sheet 1 JOIJEI- 3 1N VENTOR ORNEY June 24, 1958 H. scHoENGoLD 2,840,177

LOUDSPEAKER DIAPHRAGM SUPPORT Filed July 28, 1955 2 sheets-Sheet 2 ,37 1i l `1 b L29 MS RS cS E: 7. 5 F --LCb I-IJ gg Erg- 5- g u.| m II o z E 8INVENTOR m lm 9 O FREQUENCY INCREASE f q.; A ORNEY United States PatensLOUDSPEAKER DAPHRAGM SUPPRT Herbert Schoengold, Mount Vernon, N. Y.,assigner, by mesne assignments, to Alexander I. Abrahams, New York, N.

Application July 28, 19%', Serial No. 524,379

6 Claims. (Cl. IBIS-431) This invention relates broadly to theloudspeaker art and in its more specific aspects it relates to a novelsupport or suspension means for loudspeaker diaphragms which providesgreater unrestrained motion while having a high damping action whichproduces a substantially extended low frequency reproduction withsubstantially reduced distortion when compared with prior art support orsuspension methods; and the nature and objects of the invention will bereadily recognized and understood by those skilled in the art to whichit relates in the light of the following explanation and detaileddescription of the accompanying drawings illustrating what I at presentbelieve to be the preferred embodiments or mechanical expressions of myinvention from among various other forms, arrangements, combinations andconstructions, of which the invention is capable within the spirit andscope thereof.

My invention concerns an improvement in loudspeakers of the paper conetype and particularly to an improved suspension method permittinggreater amplitudes of excursion, better damping, reduction of rimreflections, and other attending advantages.

Normally, the maximum axial travel of a diaphragm in a conventionalloudspeaker is limited by the stiffness of the moving system, and thisstiffness or mechanical impedance is usually incorporated into thecompliant beads or corrugations at the periphery of the cone. In suchconventional loudspeakers the elastic limit of such compliance may bereached at low frequencies before the cone has moved to the ultimatepeak as required by the input or exciting current in the voice coil.This results in a limiting action accompanied by severe distortion, suchdistortion being referred to as suspension non-linearity.

The basic requirements of a linear vibrating system, i. e., one wherethe displacement is proportional to the applied force, in this casevoice coil energy, are the combination of linear compliance and a fixedmass. ln practice such systems are not simple for large magnitudes ofvibration. The compliant beads or corrugations at the edge of the conetogether with the centering spider act as a spring, but like mostsprings are linear for only a limited range of displacements.

ln accordance with this invention, greater freedom of excursion isprovided without appreciably affecting the stability of the voice coilwith respect to the air gap. Further improvements in this invention liein the damping quality of the suspension which reduces what is known inthe trade as rim hickey which is caused by reiiection from the frame orsupporting basket. Such reflections which are due to sound wavestraveling through the cone, being reflected back by the supporting framethus causing cancellation or out-of-phase conditions at specificfrequencies usually in the mid-range portion of the audiospectrum.Another contributory value of this invention lies in the damping orresistive component added to the moving system which effectively servesto reduce the undesirable distorted peak amplitude at the resonant fr-ICC quency. The combined effect of these improvements is to provide fora substantially extended low frequency reproduction containing lessdistortion than is obtained from hitherto existing suspension methods.

A loudspeaker may be considered a practical vibrating system and becomesresonant at some low frequency. At resonance the reactive components ofmechanical impedance are zero, namely, the reactances due to inherentstiffness and inherent mass are cancelled leaving an impedance which ispurely resistive. This resistive component in a conventional suspensionis small because it is due to slight friction of fibres and molecules ofthe paper stock, therefore, the diaphragm exhibits excessive sensitivityat resonance not consistent with its sensitivity at frequencies above orbelow resonance. The cone attempts to reach extreme excursions at thisresonant frequency, yet cannot do so because of the final restraints ofthe rim corrugations. Another disadvantage lies in the large rise inelectrical impedance of the voice coil at resonance causing the powerdrawn from the amplilier to be disproportionate and small compared tothe power drawn at other frequencies. This overscnsitized condition atthe resonant frequency results in distortion, boom and hangover, givinga lack of distinctiveness and coloration to bass instruments and otherlow frequency impulses.

in order to increase the resistive component and so reduce theinordinate behaviour at resonance, it has been the practice in the artto paint the rim corrugations with a compound such as Viscaloid or alatex mixture, but such damping treatments are only of very minor effectas can be shown by simple impedance and frequency response measurements.Damping by inherent characteristics of the moving system is rare and atbest is usually loaned by external factors such as the baiiie orspecialized enclosure design.

This invention incorporates a special material giving greaterunrestrained motion yet having a high damping action which reduces thepeak amplitude and high counter E. M. F. at the resonant frequency. Sucha dissipative or resistive force likewise prevents the reflection ofsound waves which travel through the cone material from being reflectedback from the supporting basket; thus it reduces or eliminates the rimhickey or cancellation effect at mid-frequencies.

The material which l have found endows a loudspeaker with many desiredand highly advantageous characteristics when used as a support orsuspension means for the loudspeaker diaphragm is one which isinherently pliable or compliant and is substantially non-resilient atleast partly because of its cellular structure, and after deformationreturns slowly to its normal configuration. Such a material having thesenecessary inherent characteristics is preferably of open cellularstructure having a mechanical inertance so that when it is compressedthe air is driven out of the cells and when released from compressionthe .air attempts to re-enter but because of the mechanical inertance ofthe material, the cells remain constricted for an appreciable time, theair being drawn only slowly back into the cells. There is thus nosubstantial motion imparted by the material itself resulting from itsown momentum or stored energy. As one example a polyvinyl chlorideopen-cell formulation will produce a plastic foam which is pliable andendowed with the aforementioned desired characteristics when used as asuspension means for the diaphragm. It is within my contemplation toalso use a closed cell foam as the suspension means for the diaphragm.

With the foregoing general objects, features and results in view, aswell as certain others which will be apparent from the followingexplanation, the invention consists in assale? fp j certain novelfeatures in designs, construction, mounting and combination of elements,as will be more fully and particularly referred to and specifiedhereinafter.

Referring to the accompanying drawings:

Fig. 1 is a view in cross section illustrating a loudspeaker having thediaphragm supported in accordance with this invention.

Fig. 2 is a detailed view in section of a modified form of my invention,with parts thereof broken away.

Fig. 3 is a detailed view in section of a further form of my invention,with parts thereof broken away.

Fig. 4 is a section through a strip of the compliant Vplastic foammaterial constituting the suspension means for the diaphragm andillustrating the cellular structure thereof.

Fig. 5 is a detailed view inV section illustrating yet another form ofmy invention, with parts thereof broken away. Y

Fig. 6 is an enlarged schematic View of a single cell of the compliantcellular material and its reactions when the diaphragm which it suspendsis at rest (a), under tension (b), and when it is under compression (c).

Fig. 7 is a graph illustrating the numerical relationship between voicecoil impedance and frequency, sometimes referred to as the motionalimpedance characteristic.

Fig. 8 is an electrical analog to the acoustic properties of thisloudspeaker. I

Referring to Fig. l of the accompanying drawings wherein I haveillustrated one form of my invention which includes a supporting basketor frame 1 for the loudspeaker which has the usual open areas for therelease of the back pressure and is provided with a conventional magnetstructure formed by a ring magnet 3, a center pole 5, a backplate orreturn keeper 7 and an outer annular pole piece 9.

The center pole piece 5 and the outer pole piece 9 leave an annular airgap in which the magnetic flux is concentrated. The voice coil winding11 wound on a cylindrical form is attached to the apex of a paper cone13, usually of conical or curvilinear shape. This cone or diaphragm 13is the essential piston generating sound. A centering spider 15 havinglarge flexibility in the axial direction but large stiffness in alateral direction serves to maintain the voice coil properly centered inthe air gap yet capable of extreme axial motion.

The cone or diaphragm 13 terminates in a peripheral edge having a fiator flanged section 17 which is formed with `a stiffening bead 19. Fixedto the inner side of flange providing section 17 by cementing or in anyother suitable manner is an annular or ring suspension element 21 whichis formed of a compliant material of open cellular structure. Thisannular suspension element 21 which is particularly illustrated in Fig.4 may be a polyvinyl Vchloride open cell formulation which ischaracterized by being deformable but following deformation anappreciable time lapse occurs before it assumes its normal shape. It isthus mechanically inert and has no tendency to impart any motion of itsown due to momentum or stored energy as would be the case with rubber orany other springy substance. The compliant material may be an open-cellfoam wherein each cell is open to adjacent cells by some form of smallpassageway, usually through a reduced or constricted opening. Thus inthe compliant material there is a random system of cells and open doorsrunning in all directions. The term open cell is used to mean a materialwhich is porous out to the atmosphere and permits the passage of air inany direction, albeit with some resistance. This resistance is at aminimum and becomes greater when under compression or tension. v

The annular suspension means 21 being'formed of a material having theaforementioned characteristics may resemble in appearance and have thefeel of foam rubber but with a porosity permitting a flow of air throughits radial width.

4 Y In order to mount the annular suspension means 21 in the combinationI fix the opposite surface thereof by cementing to the projectingsurface of a semi-rigid paper annulus 23 which is cemented to theseating flange 25 of the basket. A rim gasket 27 may be employed asshown which functions to take up small irregularities of the mountingsurface of the baflie or cabinet in vwhich the speaker is installed.

The cone 13 moves as a rigid body, the stiff edge 17 preventingeccentric'motion of the peripheral rim. The pliable material 21contributes compliance or freedom to the cone vibrations. In addition topossessing compliance by being capable of expansion and compression, thematerial of suspension number 21 performs an acoustic purpose in that itpermits passage of air through its cells. Air passing through the opencells presents a form of viscous damping to the mechanical motion of thecone. The open cell construction of the annulus 21 allows the conned airof the cabinet or bafe to ow through these cells, and since thedimensions of these openings are small the air flowing through themassumes an appreciable velocity. Since the velocity or energy in the owis a function of cone amplitude, the effect of damping of the material21 will be automatic. Since large amplitudes are encountered in theregion of resonance, a self-damping action is exerted where it is mostneeded. This is a pneumatic form of operation.

The tiny cross-sectional openings together with their random andtortuous paths create a high mass inertance and a high degree of viscousresistance. tude of mass inertance and high resistance which preventscancellation of front radiation by the rear wave. This can be explainedby reference to the analog diagram of Fig. 8 wherein the speaker isassumed to be installed in a cabinet or bale of conventionalproportions. In Fig.

' 8 Ms, Rs, Cs, represent the mass, resistance and stiffness,respectively, of the moving system. Cb is the compliance of the air inthe enclosure, M represents the mass due to the inertance of the poresof the suspension material, R is the resistance or friction of thepassage of air through this material. If M and R are small in value, asfor instance Iby large pores or open areasV then a severe shuntingeffect takes place. When M and R are high, negligible shunting exists asin the case of this invention.

Across Cb acoustic cancellation may occur between front and rear waves.By selecting M and R so that their values are relatively high, thenacoustic short-circuiting is avoided. In practice M and R are selectedin terms of material porosity and density so that the acousticcancellation will be just avoided Ifor the smallest cabinet volume withwhich the speaker is likely to be associated. Thus a balance is observedbetween the conflicting requirements of damping and acousticnon-interference.

In Fig. 2 I have illustrated a further form of my invention and haveused the same reference numerals to designate parts which are the sameas those disclosed in Fig. l. In this form of my invention I provide anannular suspension member designated in its entirety by the numeral 29and this suspension member is formed of the same compliant material ofcellular structure which has been described above in connection with thesuspension member 21. The suspension member 29 is formed to provide asupporting body portion 31 and a depending Y free portion 33 which is ofgreater thickness than the body portion 31. The radial dimension of thesuspension member or annulus 29 is greater than the same dimension ofbasket flange 25 so that the depending portion 33 will be free of andout of contact with the ange when body portion 31 is in operativeposition cemented to flange 25. The cone or diaphragm 13 is providedwith a thickened or reinforced rim 35 which is cemented to the freedepending'portion 33 of suspension member 29 to thereby suspend thediaphragm. The mounting gasket 'I5 37 is attached by cementing itagainst the body portion 31 It is this 1nagni-v of the suspension member29 vas clearly illustrated in Fig. 2. p

A further form of my invention is illustrated in Fig. 3 of the drawingsand the same reference numerals have `been used to describe similarparts of the loudspeaker. In this form of my invention I use twosuspension members, an outer suspension member 39 and an innersuspension member 41. Each of these members is, of course, of annularform, and is Iformed of a compliant cellular material such as that usedin suspension members 21 and 29. The ydiaphragm 13 is formed with aflangelike rim 43 which extends in substantial perpendicular relationwith respect to the axis of the cone or diaphragm. The rim 43 is mountedbetween compliant suspension members 39 and 41, the member 41 -being inabutment with, and, if desired, cemented to flange of basket or frame 1,and if desired the rim 43 may be cemented to the two suspension members.The outer suspension member 39 is clamped in operative position by meansof a flanged clamping ring designated generally by the numeral 45. Thisclamping ring includes a `body portion 47 which abuts against and isfixed to an extension 49 of flange 25 and also includes a dependingskirt portion 51 which clamps against suspension member 39. With thevarious elements mounted and arranged as described it will be recognizedthat an annular air space 53 is provided about the suspension membersand that this air is vented to the exterior by vent holes 55 which areprovided in seating flange 25 and the skirt portion 51 of clamping ring45. It may be noted that under operation of the loudspeaker one of thesuspension members will be in compression while the other will be intension. The usual rim gasket 57 may be `used in the organization ofthis form of my invention.

The form of my invention which I have illustrated in Fig. 5 is broadlysimilar to that illustrated in Fig. 2; however in this form 0f myinvention the reinforced rim 59 of the cone or diaphragm 13 is embeddedby molding or by other means into the free inner edge of the freedepending portion of the compliant suspension member 29.

This invention while offering a suspension having high linearity andhigh damping, yet remains original and ingenious in that its operationis related to the acoustic system in which it is enclosed and is furtherpneumatically self-controlled by the cone amplitudes.

I am aware that compliant materials have been used before; however,materials such as foam rubber frequently employed, have a highmechanical reaction, it will store energy and has negligible dissipatingquality and as a result will encourage high peaks of resonance to 'A thepoint of self-oscillation. Prior art designs may pcssess highcompliance, or damping control but these are of a fixed nature, their'behaviour is independent of the acoustical characteristic of theenclosure, and they fail to vary the degree of damping as a function ofcone amplitude.

The material which is used by me as the support or suspension means forthe diaphragm obtains its compliance or flexibility by the deformationof the minute cells which are provided, the compliance therefore is notbased on bending but principally on compression and tension orexpansion.

In the present invention the suspension member has its maximum porositywhen the cone is at rest or when undergoing relatively small vibrations.As the amplitude of Vibration is increased, this material is alternatelystretched and compressed to a degree where the porosity is reduced dueto shrinkage and change of the open cellular areas. This shrinkage ofopen cells exists for both modes. Under vibration of the cone, atextreme peaks, the cells assume an elongated or rectangularcross-sectional shape as shown in Fig. 6. Since the viscosity of a fluidor gas is related to its proximity to the walls of the container, theeffect of an elongated section is to increase the friction between themoving stream of air and the walls. Fig. 6a shows the assumed crosssection of a cell at rest; in 6b we see the deformation under tension,and in 6c under compression. The resistance to flow of a gas will begreater in the cases of 6b and 6c than in the case ot 6a. ln actualitythe deformations of 6b and 6c will be more complex because of theirrandom inter-connecting nature, but the net effect is to substantiallyincrease the resistance to flow relative to the condition of rest.

This cellular material aids in lowering the resonant frequency in that(l It adds to the effective mass of the moving system, and

(2) Its stiffness is reduced.

Both these factors have a downward shift of the resonant `l'1eq.ency.`Reference to the relationship of 1 S .fo-Zr wherein fo is the resonantfrequency, S the mechanical stiffness, M the mass,

shows we have reduced the numerator term S and increased the denominatorterm M, thus establishing fo at a lower value, which is one of theobjectives of this design.

In Fig. 7 the velocity of the cone motion is shown as a function offrequency. Curve fa represents the conc motion of a conventionalsuspension. In this curve resonance occurs at fn. When the suspensionincorporates both additional weight and compliance as in the presentinvention the resonance will move downward as shown by curve fb.

This is a very desirable lowering of the frequency of resonance.

However, if at the same time this compliant material which forms thesuspension has high mechanical resistance as in the case of thisinvention, the amplitude of the resonant peak is suppressed as in curvefc and as has been previously indicated, it reduces the distortions andperformance aberrations attributable to high resonant peaks.

It will now be appreciated that the suspension means which I havedevised for the cone or diaphragm of a loudspeaker will endow theapparatus with many highly desirable and advantageous characteristics.

I claim:

l. A loudspeaker including a frame, a vibrating diaphragm having a rimand means for suspending said vibrating diaphragm from said frame, saidmeans including an annulus of compliant material of open cellularstructure connected with said frame and fixed to the rim of thediaphragm and extending therebetween, said compliant material beingsubstantially mon-resilient and mechanically inert.

2. A loudspeaker including a frame, a vibrating diaphragm having a rimand means for suspending said vibrating diaphragm from said frame, saidmeans including a strip of deformable material of open cellularstructure connected with said frame and fixed to the rim of thediaphragm and extending therebetween, said material being deformableunder the action of the vibrating dinphragm and characterized bycompression and tension of the cells when the said material is deformed.

3. A loudspeaker in accordance with claim 2 wherein said deformablematerial is substantially non-resilient and mechanically inert.

4. A loudspeaker including a fratrie, a vibrating diaphragm having aperipheral rim and means for suspending said vibrating diaphragm fromsaid frame, said means including an annulus of compliant material ofopen cellular structure connected with said frame and fixed to theperipheral rim of said diaphragm and extending therebetween, saidcompliant material being deformable by compression and tension ot thecells thereof under the action of the vibrating diaphragm Vand air beingdriven radially outwardly of said material when the cells are compressedand stretched.

5. A loudspeaker including a frame, a vibrating diaphragm havingra rimand means for suspending said vibrating diaphragm from said frame, saidmeans including an annlus of compliant material of open cellular strucvture connected with said frame and Xed to the rim of the diaphragm andextending therebetween, said compliant material being characterized by ahigh damping action due to the cellular structure thereof whereby theamplitude and high counter E. M. F. at the resonant frequency arereduced.

6. A loudspeaker including a frame, a vibrating diaphragm having a rimand means for suspending said vibrating diaphragm from said frame, saidmeans includ- S ing an annulus of compliantrmaterial of open cellularstructure connected with said frame and lixed to the'rim of thediaphragm and extending therebetween, said cellular compliant materialafter deformation thereof being ,substantially without stored energywhereby the material returns slowly rto its normal coniguration.y

` fRefer*ences* Cited inthe Vle of this patentA UNITED STATES PATENTS2,646,853

Pocock Iuly 28, 1953 2,670,807v Brown Mar. 2, 1954 2,734,591 Olson etal. Feb. 14, 1956 Y Y FOREIGN PATENTS K 731,860 France June 6,A 1932510,707

Y Great Britain Aug. 4, 1939

